Obtaining containers by the blowing of special preforms suitably heated within a mould having a desired shape is a widely used technique in the packaging field, in particular for manufacturing bottles for beverages.
There are substantially two different techniques, simple blowing and stretch-blowing, providing for the pneumatic blowing and the concomitant mechanical stretching of the preform in the mould. In both cases, the preforms have to reach the blowing or stretch-blowing machine in a thermal condition corresponding to the softening point of the material, so as to be able to be plastically deformed within the moulds.
Softening of preforms is carried out in special ovens, which include a series of heating modules arranged in series along the path of the preforms.
In order to maximize both the thermal efficiency and the dimensions of the oven, it is desired that the preforms are introduced and made to slide within the oven as close as possible to one another.
However, this need is in contrast to the need that the heated preforms are suitably spaced for their insertion into the moulds of the blowing or stretch-blowing machine, the pitch of which, due to obvious dimensioning reasons, cannot be reduced beyond a given limit.
In conventional machines, spacing the preforms to the pitch of the moulds is carried out by handling elements, in particular distribution stars, comprising a plurality of preform gripping elements that are arranged at the same pitch of the moulds in the blowing machine or at an intermediate between the pitch of the latter and the pitch of the preforms exiting the oven. Therefore, the distribution stars are arranged in rotation at a speed greater than the feeding speed of the heated preforms, and less than or equal to the rotation speed of the blowing machine.
This speed change imposes considerable acceleration to the preforms. When considering that preforms, when they exit the oven, are not rigid, but softened, such acceleration may cause deformation, with a consequent formation of imperfections in bottles obtained after blowing or stretch-blowing operations. In fact, deformed preforms may contact the mould surface before starting the blowing or stretch-blowing process, causing an abrupt cooling of the preform at the contact point, which thus prevents it from expanding.
It also should be noticed that the diameter of the distribution stars and/or the pitch of the gripping elements located thereon is not fixed, depending on the type of the machine. This fact further increases problems related to transfer of the softened preforms from the oven to the distribution stars, which transfer cannot be performed in homokinetic conditions. When the distribution star diameter is lower and is thus spaced apart from the release elements of the preforms from the oven, the preforms will be subjected to a two-component acceleration: a tangential component for the spacing thereof at the pitch of the star, and a radial component during the transferring step from the oven to the star. This combined acceleration is particularly detrimental, often causing deformation of the preforms.